The present invention relates to a chemical heat storage system or chemical heat pump system wherein the heat of hydration reaction of calcium bromide is utilized, and more particularly to a method of recovering the heat of reaction for use in the system.
First, a description will be given of the basic principle of chemical heat storage or chemical heat pumps conventionally practiced or employed with use of CaBr.sub.2.2H.sub.2 O.
FIG. 1 is a graph showing the saturated water vapor pressure of CaBr.sub.2.2H.sub.2 O and CaBr.sub.2.H.sub.2 O and the saturated water vapor pressure of liquid water measured by the present inventors.
When CaBr.sub.2.2H.sub.2 O is heated to 200.degree. C. by a high-temperature heat source, a dehydration reaction occurs as represented by the following equation. EQU CaBr.sub.2.2H.sub.2 O (solid)+Q1.fwdarw.CaBr.sub.2.H.sub.2 O (solid)+H.sub.2 O (I)
wherein heat Q1 is 15.0 kcal/CaBr.sub.2 mol.
Thus, CaBr.sub.2.2H.sub.2 O releases water vapor, giving 1 mol of CaBr.sub.2.H.sub.2 O and 1 mol of water vapor. The water vapor exhibits a pressure of 420 mm Hg as indicated at a in FIG. 1. This water vapor pressure is equal to the saturated water vapor pressure of water at 86.degree. C. as indicated at f in the diagram. When cooled to 20.degree. C., the water vapor condenses to water of 20.degree. C. according to the following equation, evolving heat of condensation as latent heat Q3 because the vapor pressure of water at 20.degree. C. is 16 mm Hg as indicated at b in the diagram. EQU H.sub.2 O (vapor).fwdarw.H.sub.2 O (liquid)+Q3 (II)
wherein Q3 is 10.5 kcal/H.sub.2 O mol.
When the above procedure is continued after the completion of the dehydration reaction of Eq. (I), CaBr.sub.2.H.sub.2 O undergoes a dehydration reaction according to the following equation since the water vapor pressure of CaBr.sub.2.H.sub.2 O is 130 mm Hg as indicated at c in FIG. 1 and is higher than the vapor pressure, 16 mm Hg, of water at 20.degree. C. EQU CaBr.sub.2.H.sub.2 O (solid)+Q2.fwdarw.CaBr.sub.2 (solid)+H.sub.2 O (vapor)(III)
where Q2 is 17.0 kcal/CaBr.sub.2 mol.
The dehydration reaction comes to an end when 17.0 kcal/CaBr.sub.2 mole of the heat Q2 has been given.
FIG. 4 collectively schematically shows the foregoing relationship.
In the above process, the temperature of the high-temperature heat source is 200.degree. C. and that of the low-temperture heat source is 20.degree. C., but the temperature of the high-temperature heat source need not be 200.degree. C. With reference to the water vapor pressure curves of CaBr.sub.2.H.sub.2 O and CaBr.sub.2.2H.sub.2 O, the dehydration reactions of Eq. (I) and Eq. (III) take place when the temperature is higher than the temperatures corresponding to the vapor pressure (16 mm Hg in the present case) of the condensation water at the temperature of the low-temperature heat source, i.e. higher than 122.degree. C. indicated at e in FIG. 1 for the conversion of CaBr.sub.2.2H.sub.2 O to CaBr.sub.2.H.sub.2 O and higher than 156.degree. C. indicated at d in FIG. 1 for the further conversion to the anhydride of calcium bromide. Furthermore, the temperature of the low-temperature heat source need not be 20.degree. C. The dehydration reactions of Eq. (I) and (III) take place if the temperature is lower than the tempertures corresponding to the vapor pressures of water which are equal to the saturated water vapor pressures of CaBr.sub.2.2H.sub.2 O and CaBr.sub.2.H.sub.2 O at the temperature (200.degree. C. in this case) of the high-temperature heat source, i.e. lower than 86.degree. C. and 60.degree. C., respectively, as indicated at f and g in FIG. 1.
In this way, the anhydride of calcium bromide is regenerated when the dehydration reaction of CaBr.sub.2.2H.sub.2 O is utilized for chemical heat storage or chemical heat pumps.
The heat recovery process is the reverse of the reactions (I) and (III). It is a process for hydrating the anhydride of calcium bromide according to Eq. (VI) and (V) given below.
First, according to the following equation EQU H.sub.2 O (liquid)+Q3.fwdarw.H.sub.2 O (vapor) (IV)
heat of evaporation Q3 (=10.5 kcal/H.sub.2 O mol) is given to water at 20.degree. C. to produce water vapor having a low pressure of 16 mm Hg as indicated at b in FIG. 1.
CaBr.sub.2 is exposed to the water vapor obtained to cause a hydration reaction according to the following equation. EQU CaBr.sub.2 (solid)+H.sub.2 O (vapor).fwdarw.CaBr.sub.2.H.sub.2 O (solid)+Q2(V)
The hydration reaction evolves heat of hydration Q2 (=17.0 kcal/CaBr.sub.2 mol). The highest temperature obtained at this time is 156.degree. C. as indicated at d in FIG. 1.
When the CaBr.sub.2.H.sub.2 O is further exposed to water vapor with a pressure of 16 mm Hg after the completion of the hydration reaction of Eq. (V), a hydration reaction takes place according to the following equation, developing heat of hydration Q1 (=15.0 kcal/CaBr.sub.2 mol) EQU CaBr.sub.2.H.sub.2 O (solid)+H.sub.2 O (vapor).fwdarw.CaBr.sub.2.2H.sub.2 O (solid)+Q1 (VI)
The highest temperature obtained at this time is 122.degree. C. as indicated at e in FIG. 1.
FIG. 5 collectively schematically shows this heat recovery process.
Assuming that the foregoing regeneration process and heat recovery process are practiced at an ambient temperature of 20.degree. C. with no thermal input to or output from the environment, the heat balance is as follows.
Amount of heat supplied: EQU Heat Q2 of 156.degree. C.=17.0 kcal/CaBr.sub.2 mol EQU Heat Q1 of 122.degree. C.=15.0 kcal/CaBr.sub.2 mol
Amount of heat recovered: ##EQU1##
Generally in chemical heat storage or chemical heat pump systems, the higher the temperature level of the heat recovered, the higher is the value of the heat for use as thermal energy. Accordingly, it is required that the heat to be recovered in such systems have the highest possible temperature.
The main object of the present invention which has been accomplished to fulfill the above requirement is to provide a reaction heat recovery method by which heat energy can be easily obtained at a high temperature level with a high value for use.